Examples of the application of a corrective appliance for aligning and stabilizing a plurality of bone elements include procedures for correcting spinal deformities as well as procedures for the long-term biological fusion of adjacent vertebrae. The objective of corrective procedures for spinal deformities is to align and stabilize the vertebrae so as to establish a stable, balanced and pain-free spine that is centered over the pelvis in the coronal and sagittal planes. It is desirable to keep to a minimum the number of vertebrae that are fused in that case so that as much flexibility as possible is retained in the spine. However, there are instances where stabilization of the spine necessarily involves the fusion of vertebrae.
A typical procedure for aligning and stabilizing the human spinal column involves the use of aligning devices such as high-strength metal rods, sometimes referred to as fixation rods, that are secured to bone fasteners that are attached to the vertebrae of the spinal column. The metal rods, in cooperation with the bone fasteners, serve to align and stabilize the vertebrae. Particular bone fasteners that have found acceptance in bone aligning and stabilizing procedures comprise pedicle screws. After access to the vertebrae is gained through an appropriate incision, a first grouping of pedicle screws is attached to one side of respective vertebrae that are to be aligned and stabilized and a second grouping of pedicle screws is attached to the other side of respective vertebrae. The attachments of the pedicle screws to the vertebrae are made at the vertebrae pedicles. The first grouping of pedicle screws is secured to a first fixation rod and the second grouping of pedicle screws is secured to a second fixation rod. The procedure of securing the pedicle screws to the fixation rods tends to move the vertebrae into alignment with the rods and corrects any deformity in the spine.
It will be appreciated that such an aligning procedure, particularly in the case of severe spinal deformities, can create significant forces in the nature of stresses and strains both in the spinal column and in the corrective appliance. It will also be understood that it is possible for these forces to become so great that the aligning procedure does not progress satisfactorily.